1. Field of the Invention
The present invention relates to a display panel, a display device, and a terminal device, which are provided with an optical device for distributing images and are capable of displaying images towards each of a plurality of viewpoints. More specifically, the present invention relates to a display panel, a display device, and a terminal device, which can provide an excellent display quality particularly with reflective display, and can be applied preferably to high-definition and transflective types.
2. Description of the Related Art
Due to the technical developments in these days, display panels are used in various places by being loaded on various kinds of devices from large-scaled terminal devices such as monitors and TV receiver sets, medium-scaled devices such as notebook personal computers, cash dispensers, vending machines, to small-scaled terminal devices such as personal TVs, PDAs (personal digital assistances: personal information terminals), portable telephones, and portable game machines.
Particularly, a liquid crystal display device using a liquid crystal has advantages of being thin, light, small, low power consumption, and the like, so that it is loaded on many kinds of such terminal devices.
With a current display device, the same content as that of a front direction can be visually recognized even if it is observed from other directions than the front direction. In the meantime, there has been investigated a display device that is capable of visually recognizing different images depending on the directions from which the display device is viewed, and it is expected to be a display device of next generation. An example of such display device, i.e. a device that is capable of displaying different images towards each of viewpoints in a plurality of directions, may be a stereoscopic image display device.
As depicted in Japanese Unexamined Patent Publication 2004-280079 (Patent Document 1), it is necessary as a function of the stereoscopic image display device to present different images for viewpoints on the right and left sides, i.e. to present a parallactic image for both eyes on the right and left sides.
There have been many stereoscopic image displaying methods investigated as the methods for achieving such function in a concrete way. Those methods can be broadly classified into methods that use special eyeglasses and methods that do not use any special glasses. Among those, the method using the eyeglasses includes an anaglyph method that utilizes differences in colors, a polarizing eyeglass method using polarization, and the like. This type of methods cannot avoid a trouble of wearing eyeglasses substantially, so that there have actively been investigated the methods using no eyeglasses recently.
The no-eyeglass methods include a lenticular lens method, a parallax barrier method, and the like. As depicted in Japanese Unexamined Patent Publication 2004-280079 (Patent Document 1), the lenticular lens method is a method that uses a lenticular lens as a measure for separating an image for a plurality of viewpoints. In a more strict sense, to separate the image for a plurality of viewpoints means to separate the light of a plurality of pixels to each of different viewpoint directions by the lenticular lens. In the present invention, the former expression may be used to indicate the function of the latter. Regarding the lenticular lens, one of its face is formed with a plane surface, and a plurality of semicylindrical convex parts (cylindrical lenses) extending in one direction are formed on the opposite face in such a manner that the longitudinal direction of the convex parts become in parallel to each other.
In a lenticular lens type stereoscopic image display device, a lenticular lens and a display panel are disposed in this order from the near side towards the far side when viewed from an observer (user) side. The pixels of the display panel are located on a focal plane of the lenticular lens.
Further, in the display panel, pixels for displaying an image for the right eye and pixels for displaying an image for the left eye are arranged alternately. Here, a group of pixels arranged neighboring to each other corresponds to each of the convex part of the lenticular lens. With this, the light from each pixel is distributed by the convex part of the lenticular lens to the directions towards the right and left eyes, so that the right and left eyes can recognize different images from each other. As a result, the observer can recognize a stereoscopic image.
Meanwhile, the parallax barrier method is a method that uses, as an image separating device, a barrier (light shielding plate) with a large number of thin stripe openings (i.e. slits) formed thereon. Groups of pixels for displaying an image for the left eye and pixels for displaying an image for the right eye are arranged by being corresponded to the slits of the parallax barrier. As a result, the right eye of the observer (user) cannot visually recognize the pixels for displaying the image for the left eye because those pixels are blocked by the barrier. Thus, the right eye visually recognizes only the pixels for displaying the image for the right eye. Similarly, the left eye of the user cannot visually recognize the pixels for displaying the image for the right eye, and visually recognizes only the pixels for displaying the image for the left eye. As a result, the user can recognize a stereoscopic image when a parallactic image is displayed.
When the above-described parallax barrier method is devised for the first time, the parallax barrier is disposed between the pixels and the eyes, which becomes an offense to the eyes, thereby lowering the visibility. However, it becomes possible to arrange the parallax barrier on the backside of the display panel in accordance with the materialization of the recent liquid crystal display devices. Thus, the visibility has been improved.
Therefore, the parallax barrier type stereoscopic image display device is currently being investigated actively. However, the lenticular lens method is a method for changing the traveling directions of the light, while the parallax barrier method is a method for “shielding” the unnecessary light rays. Thus, the lenticular lens method has such an advantage that there is no deterioration in the brightness of the display screen generated theoretically. Therefore, the lenticular lens method is being investigated to be applied to portable apparatuses where high luminance display and low power consumption performance are considered important in particular.
Further, as another example of the device that is capable of displaying different images to each of a plurality of viewpoints, there has been developed a plural-image simultaneous-display device capable of simultaneously displaying a plurality of different images at a plurality of different viewpoints (see Japanese Unexamined Patent Publication H6-332354 (Patent Document 2), for example. This is a display that displays different images for each of the observing directions simultaneously under a same condition by utilizing the image distributing function of the lenticular lens.
This enables a single plural-image simultaneous-display device to simultaneously provide images different from each other for a plurality of users that are located in different directions different from each other with respect to the display device.
Patent Document 2 describes that the use of the plural-image simultaneous-display device allows reductions in the setting space and the cost for electricity, compared to a case where regular single-image display devices are prepared for the number of images that are to be displayed simultaneously.
As described, the display devices where an optical device for distributing the images such as the lenticular lens, the parallax barrier, or the like is provided are being investigated actively for displaying different images from each other towards different viewpoints. However, the inventors of the present invention has found and pointed out that there are various problems to be generated by just simply providing the optical device.
As a way of example, in a case of using a transflective display panel or a reflective display panel where a reflection plate with uneven structures, in other words, structures having rough surface is provided to the pixels as depicted in Patent Document 1, there may be generated an area where the display luminance is decreased partially depending on the observing positions. If the observing position is changed, display may be viewed as if it is turned dark at the areas where the luminance is deteriorated. In some cases, patterns of Fraunhofer lines may be observed by being superimposed on the image.
This change in the luminance of the display causes a problem that the observed display quality is deteriorated. The reason for causing this problem is that the reflection angle changes depending on the tilt angle of an oblique surface that forms the uneven structure, when external light converged by the lenticular lens is reflected by the uneven structure formed on the reflection plate.
Patent Document 1 therefore proposes: a method for disposing the reflection plate and the lens in such a manner that the focal distance of the lenticular lens becomes different from the distance between the reflection plate and the lens; a method for setting the oblique surface of the uneven structure so that the uneven structure reflects, for a plurality of times, the light converged by the lenticular lens; and a method for setting the uneven structure so that the probability of having an oblique surface with a certain tilt angle existing in the uneven structure within the pixels becomes uniform in the arranged direction of the cylindrical lenses.
The above-described methods for solving the problem include the image distributing device, and those methods can be preferably applied to the display panel using the reflection plate with the uneven structure. However, those methods also have the following issues to overcome.
That is, with the method for disposing the reflection plate and the lens in such a manner that the focal distance of the lenticular lens becomes different from the distance between the reflection plate and the lens, the condition of the lenticular lens serving as the image distributing device becomes different from the optimum condition for distributing images, which brings a problem of deteriorating the effect of image distribution. This problem becomes a big issue for achieving high definition for which the pixel pitch needs to be set small even though the pitch of the uneven structure is wide.
Further, with the method for setting the oblique surface of the uneven structure in such a manner that the uneven structure reflects, for a plurality of times, the light converged by the lenticular lens, the angle of the oblique surface that forms the uneven structure needs to be set as optimum. Thus, it is necessary to develop and employ a process suited for the optimization.
Furthermore, with the method for setting the uneven structure so that the probability of having an oblique surface with a certain tilt angle existing in the uneven structure within the pixels becomes uniform in the arranged direction of the cylindrical lenses, it is necessary to optimize the uneven structure for reducing influences of the unevenness in each pixel. It is hard to apply this method to a case for achieving high definition where the pixel pitch needs to be set small even though the pitch of the uneven structure is wide, and to a case where the method is to be applied to a transflective display panel in which it is relatively difficult to secure the area of the reflection plate in each pixel, since margins for optimization of the uneven structure becomes narrowed in those cases.
These problems tend to be eased by micronizing the uneven structure. However, the uneven structure is normally fabricated by using photolithography. Therefore, it becomes necessary to improve the performances of an exposing device and photoresist for achieving micronization, and to develop an exclusive process as well. Thus, it is not easy to achieve the micronization.